1. Field of the Invention
The present invention relates to a telecommunication cable having optical fibers contained in a retaining sheath. The cable is intended in particular for transmitting telephone and/or data signals at high bit rates, for example in local area networks, or for use as a trunk cable between telephone central offices, or as a distribution or branch connection cable for user lines that can convey telephone, data and/or picture signals.
2. Description of the Prior Art
European patent application No. 0,468,878 discloses a telecommunication cable including plural optical fibers surrounded by a retaining sheath with low thickness that is easy to tear. The retaining sheath is in contact with the optical fibers to grip them tightly. The space between the fibers inside the sheath can be filled with a sealing product.
The same retaining sheath disposed directly, without decoupling, over the assembly of fibers does not degrade their transmission properties. The sheath assures cohesion of the fibers to form a highly compact module, nevertheless enabling fast stripping in situ at the time of making a connection to a load unit or a splice between cables.
The optical fibers are therefore protected from contact with the surrounding environment. Because the thickness of the retaining sheath is low, the optical fibers are not subjected to stretching and compression stresses during thermal cycles, which makes manufacture of the sheath economical, especially if the sheath is manufactured during the same fabrication operation as drawing the optical fibers constituting the module, using an in-line multiple fiber drawing/cable assembly technique.
The module obtained in this way is highly compact and facilitates connecting the cable by simple identification, easy stripping, easy handling and flexibility, which is very favorable in terms of the internal organization of splicing boxes, which can therefore be optimized in terms of size and cost.
French patent application No. 2,760,540 also concerns a cable including a plurality of optical fibers tightly gripped in a sheath such as a microtube mounted very close to the optical fibers. This patent application gives rise to the problem of increased attenuation due to microcurvatures of the optical fibers confined in the sheath if the sheath is deformed by external mechanical or thermal stresses.
To prevent microcurvature of the optical fibers tightly gripped inside the sheath, whilst retaining a sheath that is easy to cut with no risk of damaging the optical fibers tightly gripped inside the sheath, patent application No. 2,760,540 teaches that the sheath should have a Young's modulus less than 200 MPa and a Shore hardness less than 90 at a temperature of approximately +20.degree. C. and a Young's modulus less than 2000 MPa at a temperature of approximately -40.degree. C. However, the above parameters are not defined: the Young's modulus could be the Young's modulus in tension or the Young's modulus in flexion, which differ by approximately 20% in the case of thermoplastics polymers, and the hardness could be the Shore A hardness or the Shore D hardness.